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    Heavy Aluminum bed too slow for autotune.

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    Tuning and tweaking
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    • NitroFreakundefined
      NitroFreak
      last edited by

      Wait, isn´t the Objet a Polyjet machine that prints with resin?
      Can you share a picture?

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      • tjhintonundefined
        tjhinton @jens55
        last edited by

        @jens55

        The build plate is 330 x 340 x 35mm. Basically solid. Top plate is a milled/ground slab of aluminum with a pocket on its bottom for the heater. The bottom half is the "lifter" plate (also aluminum) and is fixed to 3 leadscrews.

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        • Phaedruxundefined
          Phaedrux Moderator
          last edited by

          @Phaedrux said in Heavy Aluminum bed too slow for autotune.:

          https://duet3d.dozuki.com/Wiki/Tuning_the_heater_temperature_control#main

          Maybe you missed my post?

          If auto tuning fails with a message that the temperature is not rising fast enough, this indicates that either you are using too low a P value in the M303 command, or the dead time in the existing model is much too low for your heater. You can increase the dead time using the M307 command. For example, sending M307 H0 D30 will increase the dead time to 30 seconds. The actual dead time will be known after a successful auto tuning.

          Z-Bot CoreXY Build | Thingiverse Profile

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          • tjhintonundefined
            tjhinton @NitroFreak
            last edited by

            @NitroFreak

            It is. I've removed the polyjet head from a broken objet and put ... lol ... a prusa i3 mk3s extruder I had lying around on it.

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            • tjhintonundefined
              tjhinton @Phaedrux
              last edited by

              @Phaedrux

              Is that supposed to say D value or P value?

              I've set my D value to 600 and had no luck.

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              • Phaedruxundefined
                Phaedrux Moderator @tjhinton
                last edited by

                @tjhinton You may need to use an extreme value for D for the initial tuning. 99999. Once it tunes successfully once you'll have the actual D value.

                And yes, it looks like there is a typo in that section. P=D. I'll fix that, thanks.

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                • Phaedruxundefined
                  Phaedrux Moderator @Phaedrux
                  last edited by

                  @Phaedrux said in Heavy Aluminum bed too slow for autotune.:

                  And yes, it looks like there is a typo in that section. P=D. I'll fix that, thanks.

                  Actually, no. That's not a typo. The P value is the PWM value. And it's saying that having a reduced PWM value can also cause that error message to occur.

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                  • tjhintonundefined
                    tjhinton @Phaedrux
                    last edited by

                    @Phaedrux

                    Even with an extreme D value, I get "auto tune cancelled because temperature is not falling"

                    This is because the bed heater turns off, but heat is still diffusing outward toward that peripheral thermistor. There's a delay before the temp starts to drop appreciably.

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                    • tjhintonundefined
                      tjhinton @Phaedrux
                      last edited by

                      @Phaedrux

                      For reference, I'm at P1, or 100% PWM. and it's an SSR, so I wouldn't want to PWM it fast anyway for M303, right?

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                      • tjhintonundefined
                        tjhinton @NitroFreak
                        last edited by

                        @NitroFreak

                        Image of the machine.
                        objetFFF.jpeg

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                        • Phaedruxundefined
                          Phaedrux Moderator @tjhinton
                          last edited by

                          @tjhinton Well there's two parts to that. There is the PWM duty cycle and the PWM frequency. 100% duty cycle is what you should be using (and sounds like you are.) Frequency is 10hz by default I believe, which should be perfectly fine for an SSR.

                          Is there any way to move/add a thermistor to be in closer contact to the heating pad rather than just at the edge of the plate? Having a tighter feedback loop would probably solve your problem.

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                          • jens55undefined
                            jens55 @tjhinton
                            last edited by

                            @tjhinton said in Heavy Aluminum bed too slow for autotune.:

                            "There are a couple of things that will catch you right away - generally, the first layer is put down with extra bed temperature. In your case you have zero control over the bed temperature as shutting off the heater will not drop the temperature in a reasonable amount of time."
                            My understanding is that tuning catches this, assuming you do it right and set up the tune in an appropriately controlled situation - such as in an enclosed print volume.

                            No, tuning assumes that the heater has some real control, that the bed cools when you remove power and that it heats when you apply power. What you have is a giant hunk of aluminum that does what it wants and only takes slight hints from the heater.

                            If you are not doing FFF then all my comments do not apply !

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                            • tjhintonundefined
                              tjhinton @Phaedrux
                              last edited by

                              @Phaedrux

                              I could do that. And I've thought that it would fix my issue. It's not simple to do. But it would mean the edge temp of the bed isn't correct, and it's not addressing the underlying problem - I can't use a bed that's lagging. That should be possible, right? All beds lag to some extent. I'm just outside the tolerance of M303 as is.

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                              • Phaedruxundefined
                                Phaedrux Moderator
                                last edited by

                                Yes you would be trading one problem for another, but the new problem can be worked around more easily.

                                Personally, I think the heater control loop should be as tight as possible to the heat source. That allows for best control of the actual heater. In order to compensate for the lag time of heat saturation of the large plate you can simply add a wait time. The amount of time needed can be measured and should stay constant. That could be handled with a simple G4 dwell command in your startup gcode or preheat macro.

                                With RRF3 and the ability to use conditional gcode and sensor pins, you could use both thermistors in tandem and control the heater pad with one, and then wait to start the print when the plate is at the right temp.

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                                • tjhintonundefined
                                  tjhinton @Phaedrux
                                  last edited by

                                  @Phaedrux

                                  I love this idea! Thank you. Going to try the tandem.

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